1. Field of the Invention
The invention relates generally to illumination systems, and particularly to illumination systems employing one or more switchable holographic optical elements for use in illuminating an image display.
2. Description of the Related Art
Image displays are employed in projective display systems. Projective display systems are a growing technology in the market of televisions and digital monitors. Projective displays use images focussed onto a diffuser to present an image to a user. The projection may be done from the same side of the diffuser as the user, as in the case of cinema projectors, or from the opposite side. The image is typically generated on one or more "displays"--a miniature LCD device that reflects or transmits light in a pattern formed by switchable pixels. The LCD displays are generally fabricated with microelectronics processing techniques. Each pixel in the display is a region whose microelectronics processing techniques. Each pixel in the display is a region whose reflective or transmissive properties can be controlled by an electrical signal. In an LCD display, light incident on a particular pixel is either reflected, partially reflected, or blocked by the pixel, depending on the signal applied to that pixel. In some cases, LCD displays are transmissive devices where the transmission through any pixel can be varied in steps (gray levels) over a range extending from a state where light is substantially blocked to the state in which incident light is substantially transmitted. More recently, displays have also been constructed from micro-electromechanical devices (MEMs) that incorporate small movable mirrors. The mirrors, one or more at each pixel, control whether or not light is reflected into an output direction.
When a uniform beam of light is reflected from (or transmitted through) a display, the beam gains a spatial intensity profile that depends on the transmission state of the pixels. An image is formed at the LCD by adjusting the transmission (or gray level) of the pixels to correspond to a desired image. This image can be imaged onto a diffusing screen for direct viewing or alternatively it can be imaged onto some intermediate image surface from which it can be magnified by an eye-piece to give a virtual image, as for example in a wearable display.
The displays are generally monochromatic devices: each pixel is either "on" or "off" or set to an intermediate intensity level. The display typically cannot individually control the intensity of more than one color component of the image. To provide color control, a display system may use three independent LCD displays. Each of the three LCD displays is illuminated by a separate light source with spectral components that stimulate one of the three types of cones in the human eye. The three displays each reflect (or transmit) a beam of light that makes one color component of a color image. The three beams are then combined through prisms, a system of dichroic filters, and/or other optical elements into a single chromatic image beam.
Another method of generating a full color image, which eliminates the problems of combining the beams from three separate displays is to sequentially illuminate a single monochromatic display that is updated with the appropriate primary color components of the image.
The displays can be configured as arrays of red, green, and blue pixels that are illuminated by white light with arrays of color filters being used to illuminate each pixel with the appropriate color. However, generating a color image in this manner will reduce image resolution since only one third of the pixels are available for each primary color.
A significant part of the design considerations for these systems involves the choices of light sources and provisions for effective control over the relative intensities of the light sources. This control is required to allow effective color balancing during initial calibrations as well as during operation.
Holograms essentially generate predetermined wavefronts by means of diffractive structures recorded inside hologram mediums. A hologram may be used to reproduce the effects of a particular optical element, such as a lens or a mirror. In certain cases, where complex optical operations are not being reproduced, "holographic optical elements" (HOEs) may be based on simple diffraction gratings. These HOEs may be far easier and less expensive to produce than their glass counterparts, especially when the optical element is complicated or must meet stringent tolerances. HOEs can be compact, lightweight and wavelength specific which allows more flexibility in designing optical systems. HOEs may be used to replace individual optical elements, groups of elements and in some cases entire systems of conventional optical components.